A variety of techniques and instruments have been developed for use in the removal or repair of tissue in arteries and similar body passageways. A frequent objective of such techniques and instruments is the removal of atherosclerotic plaques in a patient's arteries. Atherosclerosis is characterized by the buildup of fatty deposits (atheromas) in the intimal layer (under the endothelium) of a patient's blood vessels. Very often over time, what initially is deposited as relatively soft cholesterol-rich atheromatous material hardens into a calcified atherosclerotic plaque. Such atheromas restrict the flow of blood, and therefore are often referred to as stenotic lesions or stenoses, the blocking material being referred to as stenotic material. If left untreated, such stenoses can cause angina, hypertension, myocardial infarction, strokes and the like.
Several kinds of atherectomy devices have been developed for attempting to remove some or all of such stenotic material. In one type of device, such as that shown in U.S. Pat. No. 5,092,873 (Simpson), a cylindrical housing, carried at the distal end of a catheter, has a portion of its side-wall cutout to form a hollow housing into which the atherosclerotic plaque can protrude when the device is positioned next to the plaque. An atherectomy blade, disposed within the housing, is then advanced the length of the housing to lance the portion of the atherosclerotic plaque that extends into the housing cavity. While such devices provide for directional control in selection of tissue to be excised, the length of the portion excised at each pass of the atherectomy blade is necessarily limited to the length of the cavity in the device-in turn, the length and relative rigidity of the housing limits the maneuverability and therefore the utility of the device in narrow and tortuous arteries such as coronary arteries.
Another approach which solves some of the problems relating to removal of atherosclerotic plaque in narrow and tortuous passageways involves the use of an abrading device carried at the distal end of a flexible drive shaft. Examples of such devices are illustrated in U.S. Pat. No. 4,990,134 (issued to Auth), and 5,314,438 (issued to Shturman). In the Auth device, abrasive material such as diamond grit (diamond particles or dust) is deposited on a rotating burr carried at the distal end of a flexible drive shaft. In the Shturman device, a thin layer of abrasive particles is bonded directly to the wire turns of an enlarged diameter segment of the drive shaft. The abrading device in such systems is rotated at speeds up to 200,000 rpm or more, which, depending on the diameter of the abrading device utilized, can provide surface speeds of the abrasive particles in the range of 40 ft/sec. Auth claims that at surface speeds below 40 ft/sec his abrasive burr will remove hardened atherosclerotic material but will not damage normal elastic soft tissue of the vessel wall. Auth also admits that at surface speeds above 40 ft/sec the abrasive burr will remove both hardened and soft tissue. See, e.g., Pat. No. 4,990,134 at col. 3, lines 20-23.
Not all atherosclerotic plaques are hardened, calcified atherosclerotic plaques, however. Moreover, the mechanical properties of soft plaques are very often quite close to the mechanical properties of the soft wall of the vessel. Thus, one cannot always rely entirely on the differential cutting properties of such abrasives to remove atherosclerotic material from an arterial wall, particularly where one is attempting to entirely remove all or almost all of the atherosclerotic material.
Moreover, a majority of atherosclerotic lesions are asymmetrical (i.e., the atherosclerotic plaque is thicker on one side of the artery than on the other). Since the stenotic material will be entirely removed on the thinner side of an eccentric lesion before it will be removed on the other, thicker side of the lesion, during removal of the remaining thicker portion of the atherosclerotic plaque the abrasive burr of the Auth device or the abrasive-coated enlarged diameter segment of the drive shaft of the Shturman device necessarily will be engaging healthy tissue on the side which has been cleared. Indeed, lateral pressure by such healthy tissue against the abrading device is required to keep the abrading device in contact with the remaining stenotic tissue on the opposite wall of the passageway. For stenotic lesions that are entirely on one side of an artery (a relatively frequent condition), this means that the healthy tissue across from the stenotic lesion will be exposed to and in contact with the abrading device for substantially the entire procedure. Moreover, pressure from that healthy tissue against the abrading device will be, in fact, the only pressure urging the abrading device against the atherosclerotic plaque. Under these conditions, a certain amount of damage to the healthy tissue is almost unavoidable, even though undesirable, and there is a clear risk of perforation. In some cases, this "healthy tissue" across from a stenotic lesion may itself be somewhat hardened (i.e., it has diminished elasticity); under such circumstances, the differential cutting phenomenon described by Auth will also be diminished, resulting in a risk that this "healthy" tissue may also be removed, potentially causing perforation. Thus, in today's clinical practice (balancing safety and residual stenosis), physicians rarely use Auth-type burr diameters as large as they would otherwise may prefer to utilize.
The Shturman device described in U.S. Pat. No. 5,314,438, provides a more flexible abrading device and the possibility of ultrasonic imaging of the atherosclerotic plaque and the vascular wall at the time of plaque removal, but still does not provide for the positioning of the abrading device against one side of the arterial wall (i.e., against the atherosclerotic plaque) without simultaneously contacting the opposite side of the arterial wall.
In a prior patent of Applicant's, U.S. Pat. No. 5,356,418, incorporated herein by reference (the "'418 patent"), Shturman described a rotational atherectomy apparatus with a special guide wire that can position an abrasive burr against eccentric plaque located only on one side of an arterial wall without simultaneously contacting the opposite side of the arterial wall. In Shturman's U.S. Pat. No. 5,312,427 (the "'427 Patent"), he describes the use of a two wire system utilizing a conventional guide wire and a positioning wire for laterally positioning an abrasive burr against eccentric plaque so that the abrasive burr contacts only the eccentric plaque and does not contact the opposing wall of the artery. This two-wire system utilizes a dual lumen catheter, a conventional guide wire disposed in one of the lumens, and a flexible drive shaft disposed over a special positioning wire in the other lumen. The positioning wire includes a pre-shaped positioning segment for laterally positioning the abrasive burr. When the flexible drive shaft with its abrasive burr is extended distally of the catheter and the special pre-shaped, curved section of the positioning wire is aligned with the abrasive burr the positioning segment of the positioning wire laterally deflects the abrasive device to provide selective control over the lateral position of the abrasive burr within the artery.
In another of Applicant's patents, U.S. Pat. No. 5,360,432 (the "'432 Patent"), incorporated herein by reference, Shturman describes a directional rotational atherectomy device having an abrading device formed by a thin layer of abrasive particles bonded directly to wire turns of a distal segment of a flexible drive shaft. Positioning and manipulation of the flexible abrasive segment of the drive shaft can be accomplished either by utilizing a special guide wire similar to the one described in the '418 Patent, or by utilizing a dual wire system similar to the one described in the '427 Patent.
When the abrasive device of the '427 Patent is rotated against a side of an atherosclerotic artery, torque develops which tends to twist the drive shaft and the positioning wire helically about the guide wire (i.e., the portion of the drive shaft extending distally of the catheter ordinarily is co-planar, but the torque created when the device engages stenotic tissue twists this portion of the drive shaft into a configuration that, rather than co-planar, is roughly helical). This effect becomes more noticeable when the drive shaft and the abrading device are extended further from the distal end of the catheter. This condition also occurs with the two-wire system of the '432 Patent. Similar forces attempt to twist the drive shaft and guide wire of the one-wire system of the '432 Patent and the device described in the '418 Patent.